CO-induced structural changes of supported Rh promoted by NO

The effects of NO on the CO-induced structural changes of Rh deposited on SiO2, MgO and TiO2 are investigated in the temperature range 110-503 K by means of infrared spectroscopy. It was observed that the addition of NO to CO dramatically enhanced the development of gem-dicarbonyl, Rh-I(CO)(2), from Rh-x-CO species - indicative of the occurrence of the oxidative disruption of the Rh-x cluster - for all the three samples at 233-300 K. This is explained by the formation of a strong bond between NO and Rh-x crystallites which weakens the Rh-Rh bond. It is assumed that the NO and its dissociation product, the adsorbed O, participate in the oxidation of isolated Rh-0 atoms to Rh-I ions. On the other band, the presence of NO slowed down the conversion of gem-dicarbonyl into Rh-x-CO at and above 448 K to a great extent, indicating that NO retards the reductive agglomerization of Rh-I to Rh-x crystallites on these supports.